This is an incredible recipe that may cause a few family feuds! Flavourwise, chocolate and walnuts go brilliantly together, and the marshmallows give a gorgeous, gooey consistency. Serve the pud hot with creme fraiche or ice-cream.
Into a bowl sift together the flour, ½ of the cocoa powder, the baking powder, and the salt. In another bowl whisk together the eggs, the caster sugar, butter, milk, and vanilla, add to the flour and stir the batter until it is just combined. Stir in the walnuts and marshmallows and spread the batter evenly in an ungreased 8-inch-square baking tin.
In a bowl whisk together the remaining cocoa powder, brown sugar, melted chocolate and the boiling water, pour mixture over the batter, and bake for 35 to 40 minutes. Serve the cake hot with crème fraîche or ice cream.